This application claims benefit of priority to Japanese Application No. JP 2001-26889 filed Feb. 2, 2001, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a power conversion device and more particularly relates to a power conversion device wherein control is arranged to be maintained in stable fashion by detecting occurrence of power source voltage drop with high sensitivity without any possibility of spurious detection.
2. Description of the Related Art
FIG. 1 illustrates the construction of a thyristor Leonardo device provided with a power source drop detection circuit.
In FIG. 1, a thyristor converter 2 constituted of a converter for converting AC power from AC power source 1 to any desired DC voltage and an inverter for regenerating the generated power from DC electric motor 3 constituting the load to AC power source 1 is connected to AC power source 1.
When a thyristor converter 2 is performing regenerative operation, if voltage drop or shutdown etc. of AC power source 1 occurs, there is a possibility of loss of commutation, leading to a short-circuit fault; a power source voltage drop detection circuit 4 is therefore provided in order to perform a prescribed protective action on detecting a drop in the voltage of AC power source 1.
Power source voltage drop detection circuit 4 outputs a power source voltage drop detection signal PSF_S (Power Source Fall Signal) by inputting to comparator 8 from the system of AC power source 1 an AC power source voltage signal Vac detected by a power source voltage signal detection circuit comprising transformer 5, rectifier 6 and AC/DC converter 7, and a power source voltage detection signal Vpsf (this signal means setting value).
When power source voltage drop detection signal PSF_S is activated, the thyristor Leonardo device performs a prescribed protective action.
However, in a conventional power source voltage drop detection circuit 4 as described above, there are the following problems.
The system of AC power source 1 fluctuates, influenced by the season and the operating condition of large capacity loads connected to the same system.
Fluctuations of this AC power source 1 have the following effects on power source voltage drop detection circuit 4.
For example, if the voltage of the system of AC power source 1 drops due to increase in the load of a large capacity load in the same system, the difference between power source voltage detection signal Vpsf and AC power source voltage signal Vac diminishes, with the result that the margin in respect of power source voltage detection is eliminated; if the setting of power source voltage detection signal Vpsf is raised in order to raise the detection sensitivity, there is a possibility of spurious detection.
For example, the power source voltage drop detection signal PSF_S when the detection level is taken as 85% is indicated by A in FIG. 2; however, as shown in this Figure, although occurrence of shutdown can be detected, if the voltage of the system of AC power source 1 drops by 15% or more due to power source fluctuation this results in spurious detection even if no shutdown occurs.
Also, if the voltage of the system of AC power source 1 rises due to decrease in the load of a large capacity load, the AC power source voltage signal Vac also rises, so detection of drop of the power source voltage at the original detection level becomes impossible.
Accordingly, one object of the present invention is to provide a novel power conversion device comprising excellent means capable of detecting power source voltage drop even if the power source is affected by seasonal fluctuation and/or the operating condition of a large capacity load connected to the same system, without being affected thereby.
This object is achieved by a power conversion device having the following construction.
Specifically, a power conversion device according to the present invention that converts power from a power source to any desired voltage, comprises:
averaging means (unit) for obtaining an average power source voltage signal by averaging the power source voltage signal; and
power source voltage drop detection means (unit) that compares the average power source voltage signal obtained by this averaging means (unit) and the instantaneous value of the power source voltage signal and outputs a power source voltage drop detection signal on detection of drop of the power source voltage by this deviation becoming more than a fixed value.
With the present invention, spurious detection of power source voltage drop produced by the effects of power source fluctuation such as seasonal fluctuation can be prevented.